1. Field of the Invention
The present invention relates generally to the field of antennas, and more specifically to low-profile, conformal, broadband platform-mounted antennas.
2. Description of the Related Art
The descriptions and examples included herein are not admitted to be prior art by virtue of their inclusion in this section.
A wide range of frequencies is currently used in military and commercial communications, from about 3 MHz to about 3 GHz. Although much of commercial cellular telephone communication uses frequencies of about 800 MHz and above, the lower-frequency portion of the above range is very important for applications including military and public safety communications. Commercial pagers also operate at a relatively low frequency of about 150 MHz. Advantages of lower frequencies include improved diffraction around and penetration through obstacles such as walls and foliage, and reduced path loss and attenuation in air, resulting in longer transmission lengths for a given power level. The frequency range from 3 MHz to 30 MHz, designated as the "high-frequency" (HF) communications range, and the 30 MHz to 300 MHz range, called the "very-high-frequency" (VHF) range, are of interest for the lower-frequency applications described above.
Wavelengths in the HF and VHF ranges are on the order of meters to tens of meters. For communications in these ranges, and particularly for mobile communications, it is thus generally necessary to utilize electrically-small antennas, or antennas with geometrical dimensions which are small compared to the wavelengths of the electromagnetic fields they radiate. Unfortunately, electrically-small antennas exhibit large radiation quality factors Q; that is, they store (on time average) much more energy than they radiate. This leads to input impedances which are predominantly reactive and in turn allows the antennas to be impedance-matched only over narrow bandwidths. Furthermore, because of the large radiation quality factors, the presence of even small resistive losses leads to very low radiation efficiencies. In particular, the radiation Q of an electrically-small antenna is roughly proportional to the inverse of its electrical volume, and is essentially inversely proportional to the antenna bandwidth.
It is desirable to communicate over broad frequency ranges, particularly for military communications, in which a wide range of frequency bands is used. Furthermore, military communications may involve bandwidth-intensive techniques such as frequency-hopping to avoid interception and jamming. The above-described constraints on electrical size vs. band-width suggest that physically large antennas and/or multiple antennas are needed to cover a broad frequency range, or large bandwidth, at low frequencies. In practice, large antennas in the form of tall, high-profile whips are frequently used for mobile communications at HF and VHF frequencies. These whip antennas have disadvantages, however. Their high profile makes the antennas observable and relatively fragile. In military applications, high observability is disadvantageous because communications systems are high-priority targets for an enemy. Even in non-combat situations, whip antennas can be damaged during travel in forested terrain, for example. The use of multiple relatively narrowband antennas in order to cover a broad bandwidth is undesirable because it increases system complexity. It would therefore be desirable to develop a low-profile, broadband electrically small antenna.